The invention relates to control systems for appliances. Specifically, the invention involves a control system that provides high precision temperature control.
A conventional household oven allows a user to set a temperature for baking or cooking food. The oven heats an oven chamber to the desired temperature and attempts to maintain that temperature in the oven chamber for the duration of the cooking period. To heat the oven and maintain the oven temperature, the conventional household oven includes heating elements, a temperature sensor, and a controller. For the oven's basic operation, the heating elements are supplied with power to heat the oven chamber. The temperature sensor supplies a signal that indicates the temperature within the oven chamber. When the temperature sensor indicates to the controller that the temperature within the oven chamber reaches the desired temperature, the controller removes the power from the heating elements. The controller later applies additional power to the heating elements when the temperature sensor indicates that the temperature within the oven chamber falls below the desired temperature.
A common control method for maintaining the temperature within the oven chamber is thermostat-style trip point detection with hysteresis. With this method, the heating elements are provided with power until the temperature sensor reads an upper trip point temperature. Typically, the heating elements are driven fully on using relays. Then, the heating elements are turned off using the relays and remain off allowing the oven chamber cools until the sensor reads a lower trip point temperature. Once the lower trip point temperature is reached, the heating elements are again energized repeating the method. For the typical household oven, the temperature sensor is a standard resistive temperature device (RTD) sensor. The temperature sensor is typically mounted in the corner of the oven chamber. The temperature sensor supplies the signal to the controller that reads the oven temperature with a precision of about two degrees Fahrenheit.
One shortcoming of the thermostat-style trip point detection method is that the temperature at the center of the oven may vary significantly. Because the temperature sensor is located on the cavity wall of the oven chamber a lag in the temperature reading occurs. The center of the oven chamber can experience large temperature swings while the temperature sensor reads only small changes in temperature. Improving the performance of the thermostat-style trip point detection method requires better thermal isolation of temperature sensor or a higher precision measurement of the temperature sensor. However, these solutions add significant costs to the oven appliance.
Thus, there is needed a control system for maintaining the temperature within a small range within the oven chamber. The system should use a simple temperature sensor and cost-effective electronic components that do not depend on small fluctuations in the temperature sensor input to determine when to apply heat.